Distinctive phases and variability of vibration-induced postural reactions highlighted by center of pressure analysis

Background The vibration-induced postural reaction paradigm (VIB-PR) offers a unique way for investigating sensorimotor control mechanisms. Measures of VIB-PR are usually calculated from the whole VIB period, yet recent evidence proposed that distinctive mechanisms are likely at play between the early vs. later phases of the postural reaction. Objectives The present work verified if spatiotemporal analyses of center of pressure (COP) displacements can detect differences between these early/later phases of VIB-PR. Also, we further characterized the intra/inter-individual variability of COP measurements, since the underlying variability of VIB-PR remains largely unexplored. Methods Twenty young volunteers realized two experimental conditions of bipodal stance with eyes closed: (i) bilateral VIB of tibialis anterior (TIB) and (ii) Achilles’ (ACH) tendons. Each condition consisted of five trials and lasted 30 s as follows: 10 s baseline, 10 s VIB and 10 s post-VIB. Linear COP variables (antero-posterior (AP) amplitude & velocity) were computed for both VIB and post-VIB periods using the following time-windows: early 2 s, the later 8 s and the whole 10 s duration. Intra- and inter-individual variability were respectively estimated using the standard error of the measurement and the coefficient of variation. Both variability metrics were obtained using five vs. the first three trials. Results Significant contrasts were found between time-windows for both VIB and post-VIB periods. COP variables were generally higher during the early 2 s phase compared to the later 8 s phase for both TIB [mean difference between 8 s– 2 s phases: Amplitude AP = -1.11 ± 1.14 cm during VIB and -2.99 ± 1.31 during post-VIB; Velocity AP = -1.17 ± 0.86 cm/s during VIB and -3.13 ± 1.31 cm/s during post-VIB] and ACH tendons [Amplitude AP = -0.37 ± 0.98 cm during VIB and -3.41 ± 1.20 during post-VIB; Velocity AP = -0.31 ± 0.59 cm/s during VIB and -3.89 ± 1.52 cm/s during post-VIB]. Most within- and between-subject variability scores were below 30% and using three instead of five trials had no impact on variability. VIB-PR patterns were quite similar within a same person, but variable behaviors were observed between individuals during the later phase. Conclusion Our study highlights the relevance of identifying and separately analyzing distinct phases within VIB-PR patterns, as well as characterizing how these patterns vary at the individual level.


Introduction Major comments:
No major comments for this section. In general, the introduction provides a nice overview of the area and justification for the work.
Line 57 -spelling mistake -should be "ageing", instead of "aging" Line 75 -I would suggest replacing ", that consisted" with "and consisting" to make it clear that the two points are related to the 2-sec period, before you discuss the 8-sec period.
Line 77 -should be "patterns" instead of "pattern" Line 98-102 -what were your hypotheses for the variation data? Please include.

Major comments:
I do have one general question about the method adopted within this paper. You state in your introduction that VIB can simulate postural responses similar to falling etc. -if this is the case, why do you perform the vibration for such a long period? If this were to be related to falls, the stimulus and response to a fall would be almost instantaneous, so why would you be interested in seeing the response to the vibrations over longer periods than this?

Minor comments:
Line 107 -"and" should be "or" to detail the fact that participants had no history of disorders or injuries, as opposed to the two co-occurring.
Line 110-111 -can you put the units with the corresponding data, as opposed to the descriptor?
Line 110 -this is body mass (kg), not body weight (N).
Line 117 -"were performed" as opposed to "were done".
Line 117 -was the same rest period given between conditions?
Line 118 -what do you mean by along the body? Arms by their sides? Hanging freely?
Line 119-120 -What was the feet position?
Line 121 -can you be more specific on the locations of the vibration devices? How did you standardise these between participants?
Line 130 -can you be more specific? What were the linear COP variables?
Line 140 -"(REF)"? Is there supposed to be a supporting reference here? Please either add the appropriate reference, or remove.
Line 161 -what pairwise comparisons did you run?

Major comments:
Throughout this whole section, you hardly report any actual results. Throughout, please provide more detail, including actual data (measured values, group mean differences etc.), and the results of your statistical tests.
In my opinion, figures 3 and 4, where you present variability data, should not be presented as bar charts -even where data are normally distributed. Bar charts arbitrarily assign importance to the height of the bar only. Whilst this is obviously an important factor, what it fails to show is how the difference between the means compares to the range of observed values. Using either a dot-plot, or combining a dot-plot with your bar chart would better display this data and paint a "fuller" picture of this variability data.

Major comments:
No major comments for this section.

Minor comments:
Line 242 -In the second and third sentences of your first paragraph, can you provide a statement of the importance or relevance of these findings?
Line 258 -can is a strong statement to make when you do not know for sure. Please use "may" instead.
Line 270 -reference is in different format to the remainders. Please incorporate the reference number.
Line 301-303 -Is this truly the case? According to your figures, the average COP position changed by ~3cm, whereas the average adult foot would be nearer 25-30cm? It would have been nice to see a figure visualising where on the foot the COP is positioned. If you have the capacity to show such a figure with this data, I would recommend including, but I appreciate that it may not be possible with your available dataset (maybe something to consider if you perform similar work in the future).
Line 314-316 -plastic adaptations indicate that there is some sort of permanent change. Is there evidence to suggest that this would be the case?
Line 317 -no previous definition of CNS -please write out in full.
Line 329-332 -is this something you are looking to address in the future? If so, include as a future direction.
Line 339 -what do you mean by anteroposterior plan? I assume this is a typo?